1. Field of the Invention
The present invention relates to a volume phase type hologram and a method of manufacture thereof.
2. Related Background Art
Gelatin sensitized by a dichromate has hitherto been widely known and used as a recording medium for volume phase type holograms. Another kind of recording medium is known for forming a volume phase type hologram comprising a hydrophobic polymer, exemplified by poly(N-vinylcarbazole), (hereinafter referred to as PVCz), sensitized by addition of an iodine compound, and volume phase type holograms recorded in the above recording mediums are also known, which are disclosed in detail in prior art publications such as U.S. Pat. No. 4,172,724, U.S. Pat. No. 4,258,111 and U.S. Pat. No. 4,287,277.
The volume phase type hologram is employed for recording the interference pattern of laser beams in a thin film of 20 .mu.m thick or less. When using a visible light laser, the layer of photosensitive material for hologram recording is required to be fixed with positional accuracy of approximately from 0.01 to 0.02 .mu.m during exposure in order to record a Bragg's lattice of approximately 0.1 to 0.2 .mu.m interstice within a film.
Further, in the treatment subsequent to the exposure, the photosensitive recording layer is swollen and then shrunk to modulate the refractive index of the portion recorded by laser beam, where the swelling and then the shrinking of the recording layer are in a direction substantially perpendicular to the recording layer. Because of this restriction both in light exposure and in after-treatment, conventional volume phase type holograms are tightly fixed on and supported on a rigid substrate plate.
The above-mentioned conventional methods involve a disadvantage in that a diffraction lattice cannot be formed satisfactorily in the recording layer in proximity to an interface of the substrate, because the recording layer is supported and fixed by the substrate surface and therefore a solvent cannot penetrate sufficiently to cause the necessary swelling and shrinking even though a latent image has been formed in the recording layer near the substrate by exposure.
To offset this disadvantage, the use of a solvent having a high swelling power may be considered. However, such a solvent weakens the fixation of the recording layer to the substrate, and makes it impossible to restrict the direction of swelling and shrinking of the recording layer only to the thickness direction; thus the formation of a desired diffraction pattern becomes difficult. To solve this problem, Japanese Patent Laid-open Application, Publication No. 62-502289 (1987) proposes a method in which the photosensitivity at the recording layer is reduced in the vicinity of the interface to the substrate so as to retard the formation of a lattice image near the interface. In this instance, the region which practically serves as a hologram in a limited thickness of the recording layer decreases.